We'll go down this road 'til it turns from color to black and white
The color change implies:
They’ll grow old together (black hair turns gray)
They will escape to a bygone era (like black and white movies). They will stay together until it becomes like an old movie.
It could also refer to the film / book Wizard of Oz which switches from color back to black and white after Dorothy journeys down the yellow brick road and returns to Kansas.
Basically, this line also brings the song to a full circle; refer back to the first line about the tornado.
He’s saying he wants to spend his life with this person, to stop dwelling in his dreams and make them a reality.
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